Recently, digital cameras are widely used which convert images, captured by a solid state imaging device such as CCD, into digital image data and store the data in recording media, such as an internal memory and a memory card. In order to facilitate focusing light rays on light receiving elements, such as photodiodes arranged in a matrix, the solid state imaging device in such digital cameras is generally equipped with microlenses located above the light receiving elements.
FIG. 7 illustrates a structure in cross section of a conventional CCD. In FIG. 7, a CCD 100 includes a semiconductor substrate 102 with photodiodes 101 formed therein, a transparent insulating film 103 on a top surface of the semiconductor substrate 102, transfer electrodes 104 next to each line of the photodiodes 101, a light shielding layer 105 to cover the transfer electrodes 104 for light protection and having openings 106 above each photodiode 101, a planarizing layer 107 with a flat top surface for covering over the light shielding layer 105 and the photodiodes 101, a color filter 108 on the planarizing layer 107, and a microlens array 109 above the color filter 108 aligned such that microlenses 109a are located above each photodiode 101. In the CCD 100, a light ray is converged by the microlens 109a and received by the photodiode 101, which generates an electric charge proportional to the amount of light received. The generated electric charge is then transferred as a signal charge in a vertical direction by the transfer electrodes 104. The microlens array 109 is a transparent material layer on the color filter 108 and has microlenses 109a shaped by, for example, a dry etching technique.
Unfortunately, the present-day dry etching techniques hardly put all the microlenses 109a into exactly the same shape. As a result, an efficiency for focusing light ray (i.e., focusing efficiency) varies among the microlenses 109. Additionally, the solid state imaging devices are becoming even smaller yet hold more pixels in these days, and the photodiodes are getting smaller in dimension. There is therefore a need to focus incident light rays effectively on the photodiodes. Accordingly, Japanese Patent Laid-open Publication No. 2001-44406 discloses a solid state imaging device which has a condensing lens directly above a light receiving part. Furthermore, Japanese Patent Laid-open Publication No. 2000-150845 discloses a solid state imaging device which has a well-form trench structure that directs a light ray from a color filter to a light receiving part. These components serve to prevent deterioration of the focusing efficiency due to variation in shape of the microlenses.
However, even these solid state imaging devices cannot provide sufficient focusing efficiency and sometimes cause smear noise that appears as a vertical whitish line on a captured image. Rightly, an incident light ray from the microlens 109a should be directed to the opening 106 of the light shielding layer 105 and reach the photodiode 101. Insufficient focusing, however, allows some of the incident light rays to go toward the light shielding layer 105. Hardly does the light shielding layer 105 provide an absolute light shielding function, such an incident light ray, if it is intense enough, can permeate the transfer electrode 104 and reach a charge transfer section 110 where the incident light ray is converted into an electric charge. This electric charge saturates the signal charge in the charge transfer section 110 and causes the smear noise.
Another disadvantage of the Japanese Patent Laid-open Publication No. 2001-44406 is that the condensing lens above the light receiving part has a lower refractive index than the surrounding planarizing layer. This configuration makes the light rays proceeding form a flattening, or planarizing, layer toward the condensing lens more likely to diffuse and may possibly deteriorate the focusing efficiency. Another disadvantage of the Japanese Patent Laid-open Publication No. 2000-150845 is the formation of the well-form trench structure that extends from the microlens toward the light receiving part. Such a trench structure is very difficult to form in modern solid state imaging devices that are so small in dimension. As a result, voids and other defects are more likely to occur, and effective focusing of light rays on the light receiving part is hardly achieved.